The present invention relates to an offset duplicator. In particular, the invention relates to an offset duplicator which is capable of process color offset printing.
The printing industry has reached high levels of sophistication over the past several years as many businesses and other fields have placed increasingly greater demands on the quality of their printed materials. From the industry's early beginnings in straightforward black-and-white reproduction, printing has followed a gradual development in terms of techniques, quality, and output which has led to the current state of the printing art. Users of printed materials cover a wide range of interests and fields. Virtually every commercial and noncommercial entity uses the print media for a variety of purposes.
In order to satisfy the need for printed materials a wide variety of printing apparatus has been developed. Such equipment has typically been developed to satisfy particular printing needs. Thus a typewriter may be used to provide a single, high quality copy, but it would not generally be used if a number of copies were required. In such cases, a photocopier would typically be used to provide a relatively limited number of high quality copies, whereas a mimeograph machine or a spirit copier might be used to provide a relatively limited number of less expensive, but poorer quality, copies.
When large numbers of high quality copies are required, some type of offset printing equipment is typically used. The features desired by a particular user largely determine the particular type of printing equipment required for use on a job. Thus, a user of a large number of copies, who did not need to reproduce full color items, would conventionally use an offset duplicator.
As used herein, the term "full color" refers to items which include arbitrary colors, such as photographs, rather than items which may include a plurality of colors, such a line copy. While line copy may be printed in multiple colors, the separation processes required for "full color" or "process color" printing are not needed.
When factors such as printing in full color, cost per copy, number of copies per plate, and speed of operation are critical, as they are in a large printing operation, the type of equipment available to perform the required services becomes limited, and the cost of such equipment becomes quite large. For example, to fill the need for printing a large number of items in full color, the only printing equipment heretofore available has been the rotary offset printing press. Such equipment is capable of producing a fine quality product and is the type of equipment which is generally used to print full color.
Rotary offset color presses of the type in common use include many features which insure that they will have a very high quality product. Unfortunately, these items result in a very high acquisition cost. By way of example, in order to be able to spread a thin layer of ink onto the printing plate used in such a press, a so-called "tower" is located over each plate cylinder the press. Within that tower, there are typically more than fifty ink rollers. Their purpose is to thin out the viscous ink used in printing. A very thin layer of ink is required on the printing plate, because the plate used for each color is comprised of a very dense arrangement of dots. Those skilled in the art recognize that in a full color or "process color" printing press, the original item to be reproduced first undergoes a "color separation" procedure in which the colors of the original are separated into constituent colors which, when recombined, result in the colors of the original. Each of the constituent colors is printed separately. The physical separation is accomplished through the use of a screen having a very dense pattern of dots. The higher the dot density, the closer together the different colored dots will be on the ultimate print, and consequently the higher the resolution of the ultimate print.
Thus, the rotary offset color presses heretofore known have had to include a mechanism for insuring that when a high dot density screen is used in preparing the printing plates, i.e. one typically having from about 150 to about 200 dots or "lines" per inch, the ink applied to the plates will not fill in the spaces between the image dots. As should be obvious, if the ink layer were not applied to the plate as a very thin film, a muddled print would result.
Naturally, the preparation of the plates with the high resolution described above, the so-called "high etch" plates, involves a very exacting and precise process. This further increases the expense of producing high quality printed output on an offset press.
An additional feature of the conventional rotary offset color press is the use of so-called "transfer cylinders" to move sheets of paper from one print head to another. The transfer cylinders insure very accurate registration of sheets of paper at different locations within the press. Such registration is required in order to insure accurate color reproduction.
Unfortunately, there have been a variety of applications in which color printing has heretofore been desirable, but too costly, due to the cost of conventional color offset printing presses. The user who has not needed absolutely accurate color reproduction could not heretofore find a machine capable of process color offset printing, providing relatively good color, simple operation, and inexpensive purchase and maintenance costs.
In addition, the cost of setting up a job for a conventional rotary offset color press has been so high that it has not heretofore been economical to print small jobs in full color. Thus, the user who needs a relatively small number of color copies, i.e. less than about 2500 copies, has typically found it too expensive to have the job run by a print shop.
Instead, the user of a relatively small number of copies has been limited to a much less expensive device, such as an offset duplicator, and has had to give up color reproduction. The term "duplicator" is used herein to refer to printing equipment of the type described more fully in U.S. Pat. No. 2,821,911 entitled INTERRUPTER FOR ROTARY OFFSET PRINTING MACHINE; U.S. Pat. No. 2,846,220 entitled SHEET FEEDER FOR PRINTING PRESS; U.S. Pat. No. 2,859,692 entitled SHEET DELIVERY MEANS FOR ROTARY OFFSET PRINTING PRESSES; U.S. Pat. No. 2,890,884 entitled MULTIPLE SHEET EJECTING MECHANISM; U.S. Pat. No. 2,899,202 entitled OFFSET PRINTING MACHINE AND SHEET GAGE; U.S. Pat. No. 2,915,970 entitled INKING AND DAMPENING MEANS FOR AN OFFSET PRINTING MACHINE; and U.S. Pat. No. 2,929,321 entitled INK FOUNTAIN ROLL. Each of the foregoing U.S. patents is incorported herein by reference.
While an offset duplicator is an offset printing apparatus, and it uses many of the same principles as the rotary offset color press described above, it is not designed for process color printing. Thus, while more than one color may have heretofore been printed on a single item using an offset duplicator, as described more fully in U.S. Pat. No. 2,845,860 entitled TWO-COLOR OFFSET PRINTING PRESS, full color printing on an offset duplicator has not been accomplished heretofore. In other words, the different colored inks were not separated by a color separation process, because an offset duplicator is inherently incapable of providing the fine ink layer on a plate and the accurate sheet transfer that the rotary offset color press is designed for. Heretofore, no one had designed a color separation printing process which could be used in an offset duplicator to bring its simplicity, and lower price, to the public.